The walls of a refrigerated storage room include a $4cm$ external wall with $k=1\mathrm{.}0\frac{W}{m}K,$ and a $3cm$ external wall with $k=1\mathrm{.}5\frac{W}{m}K.$ The insulation layer between the two walls is $5cm$ with a $k=0\mathrm{.}03\frac{W}{m}K.$ The inside of the room is maintained at $5C,$ and outside temperature is $25C.$ If the heat flux through the wall is $10\frac{W}{m^2},$ estimate the following:

a$.$ The overall heat transfer coefficient

$(15$

b$.$ The total thermal resistance for the wall

$(10$

c$.$ If the temperature on the inside wall surface is $7C,$ estimate the convection heat transfer coefficient on the inside wall.

Extra Credit: Compute the temperature the interface between the external wall and the insulation.